In the microfabrication of devices, it is necessary to form grooves, holes or other indentations in substrates. For example, U.S. Pat. No. 4,791,436 to Chan et al., assigned to the assignee of the present application, teaches formation of grooves or serrations in the interior orifice bore of a nozzle plate in an ink jet printer. The desired features are provided by electroforming the nozzle plate on a mask having a sculptured or grooved outer surface area.
Ever-present goals in microfabrication include achieving a high throughput and achieving a high yield. Laser ablation is an attractive option in the industry. Particularly for fabrication of ink jet components, excimer laser ablation provides a number of advantages. However, excimer laser ablation systems typically use a single-aperture projection mask, so that there must be a mechanism for providing relative movement between the projection mask and the workpiece after each ablation. This places limitations on production throughput.
To improve a high throughput in thermal ink jet manufacturing, a large-area mask may be used. Such a mask allows many orifices or channels to be ablated simultaneously. For example, ablation of all of the features in a field of view of 25 square millimeters or more may be accomplished at a single time. Moreover, excimer laser ablation using a projection mask offers the advantage that a reduction mask may be used to limit the optical power density on the mask, thereby reducing the risk of mask damage.
One approach to production of large-area masks is to machine the mask in a free-standing sheet of metal. The problem with such an approach is that the masks are particularly susceptible to the detrimental effects of localized heating generated by impingement of the mask by laser energy. The resolution of the mask may be suitable for applications such as fabrication of vias in printed circuit boards which allow dimensional tolerances of 10 microns or more. However, the masks are inadequate for production of thermal ink jet components which require resolutions of approximately 1 micron.
Another approach to production of large-area masks is taught in U.S. Pat. No. 4,661,679 to Pardee. The patent teaches a semiconductor processing technique for transferring a circuit pattern without use of a pass-through mask. Excimer pulsed ultraviolet laser radiation is directed at a mirror having the circuit pattern thereon. An incident ray which strikes an anti-reflective portion of the mirror is absorbed into the mirror. On the other hand, an incident ray which strikes a laser-reflective portion of the pattern on the mirror is reflected to the semiconductor wafer. In this manner, the circuit pattern is transferred from the mirror to the semiconductor wafer. To reduce the distortion of the mirror due to incident energy induced heating, a number of tubes are positioned on the back side of the mirror opposite to the incident side of excimer pulsed ultraviolet laser radiation. The tubes may be used to conduct pressurized gas or liquid. The patent teaches that the mirror mask is superior to pass-through masks since pass-through masks suffer from distortion due to incident energy induced heating, while the mirror mask can employ backside cooling to control the surface temperature. Backside cooling in pass-through masks is difficult since cooling tubes would interfere with the desired pattern. On the other hand, the Pardee mask does little to address the distortions due to the difference in the thermal expansion of the laser-reflective portion and anti-reflective portion of the mirror. An alternative embodiment taught in the patent is to space apart the laser-reflective portion from the anti-reflective portion by a layer of refractive material. However, this embodiment does little to cool the laser-reflective portion and it is more difficult to etch the circuit pattern on the refractive portion than it is to manufacture conventional masks.
U.S. Pat. No. 4,842,677 to Wojnarowski et al. teaches a pass-through mask for excimer laser patterning. The mask comprises a quartz substrate on which is disposed a desired patterning in the form of a reflective metal. Desirable metals for production of the mask include aluminum and silver. The Wojnarowski et al. patent does not address the detrimental effects of localized heating of the mask by the excimer laser.
It is an object of the present invention to provide an apparatus and method for optically transferring a pattern from a large-area mask to a substrate, wherein the apparatus and method do not suffer from technical limitations imposed by cooling requirements of the mask and wherein the production of the mask is not made difficult.